1. Field of the Invention
The invention relates to a gantry conveyor apparatus and, more specifically, a gantry conveyor apparatus having multiple lift tubes.
2. Description of the Related Art
Gantry conveyor systems used in the manufacture and assembly of automotive vehicles are well known in the art. Gantry conveyor systems are used as material handling devices to pick up components, such as vehicle frames, at one station and transport the components to a subsequent station along an assembly line. A gantry system generally comprises an elevated gantry beam and a plurality of gantry carts movably mounted to roll along the beam between assembly line stations. Each gantry cart supports a lift tube assembly. The lift tube assemblies are each designed to pick up a part at a station then raise the part from the station to an elevated transport position. Once in the transport position the attached part is carried by the gantry cart along the gantry beam to a position above a subsequent station along the assembly line. Once positioned above the subsequent station the lift tube lowers the attached part to the subsequent station and releases it there. In the prior art, each lift tube assembly includes its own dedicated electric or hydraulic motor that raises and lowers the lift tube on that assembly. The inclusion of an electric or hydraulic motor to drive each lift tube assembly results in a relatively complex and costly system.
FIG. 1 shows a typical prior art gantry system 10. Gantry legs 11 are used to support a gantry beam 12 above the shop floor. Typically, the gantry beam 12 is located 8 to 13 feet above the floor. Lift tubes 13 are mounted on the gantry beam 12 and are powered by a motor 14 to translate back and forth along the beam 12 to pickup and place parts at workstations on the shop floor. As shown, the gantry system 10 would service four workstations, one located beneath each of the lift tubes 13, and one located to the right of the lift tube that is furthest right on the gantry beam 12. Tooling such as a component gripper (not shown) is attached to a mounting plate on the lower end of each lift tube 13. The tooling is used to secure a part for transfer from one workstation to the next. In hemming applications, the tooling may also be used to secure a part in the hemming die for hemming. A separate motor (not shown) is provided for each of the lift tubes 13 to raise and lower the lift tubes.
Gantry systems are known to employ counterbalance systems to reduce the effective load that the lift tube motors must lift. Some gantry systems use mechanical counterbalance weights. Other gantry systems use air (pneumatic) counterbalance systems. In each case, components of the counterbalance system travel with each gantry cart. For example, in a mechanical system, a counterbalance weight is coupled with each lift tube on each gantry cart.
In air counterbalance systems, a surge tank and a pneumatic counterbalance cylinder travel with each cart. The purpose of each surge tank is to increase the volume capacity of the pressurized side of its associated pneumatic counterbalance cylinder. The increase in the volume capacity reduces the pressure differential when a counterbalance cylinder is in different working positions. It""s typical for the volume of a surge tank in such a system to be 10 times the maximum volume of its associated counterbalance cylinder. Each such air counterbalance system requires a feed line and an air regulator to supply air to make up for any losses that may occur in the system because of leaky connections and worn seals. Air counterbalance systems also have the disadvantage that the pneumatic levels and flow rates in the various parts of such systems are difficult to balance once the original factory set balance has been altered.
It would be desirable to reduce the complexity and, the cost of gantry conveyor lift systems.
The invention is embodied in a gantry conveyor apparatus for transporting a plurality of components between workstations in an assembly line. The apparatus includes an elevated gantry beam and at least two gantry carts movably mounted to roll along the beam. The apparatus also includes a conveyor that is coupled to each gantry cart and is configured to move the carts along the gantry beam between stations in an assembly line. A lift is supported on each gantry cart and is configured to engage a part at one station, raise the part to an elevated transport position, and lower and release the part at a subsequent station.
The gantry conveyor apparatus also includes a harmonic lift drive coupled to the lifts and configured to drive the lifts using a single motor. A gantry conveyor apparatus constructed according to the invention is, therefore, able to move parts using a single lift motor rather than requiring a separate motor at each lift.
Objects, features and advantages of this invention include a harmonic drive system for a gantry in which multiple lift tubes can be powered by a single motor, in which the cost of the gantry is reduced through the use of a single lift drive for the entire gantry system; in which a simple mechanical or pneumatic counterbalance can be used; in which a passive lock-up is provided for the lift tubes in the raised position while the gantry carts travel between work stations; and in which a passive lock-up can be designed to use the existing lower horizontal guide rail that guides the gantry carts.